Rename exercises dir

Name it instead "topics" so it won't be
strange if some topics included "exercises" directory.

topics/terraform/exercises/launch_ec2_instance/exercise.md

@@ -0,0 +1,12 @@
+# Launch EC2 instance
+
+## Reuqirements
+
+* AWS account
+
+## Objectives
+
+1. Write Terraform configuration for launching an EC2 instance
+2. Run the commands to apply the configuration and create the EC2 instance
+3. What happens if you run again `terraform apply`?
+4. Destroy the instance you've created with Terraform